It is known that olefins such as ethylene can be polymerized by means of a solid catalyst which comprises: a compound of a transition metal such as titanium in the trivalent or tetravalent state, and a co-catalyst comprising a reducing agent, such as an organo-metallic or, more specifically, an organo-aluminum compound.
Although these catalytic systems have an attractive degree of activity, when polymerization is concluded they generally result in the formation of polymers containing more than 100 parts per million by weight of transition metal. For most of the uses of such polymers, this makes it virtually essential to remove the catalytic residues by a special treatment.
It is also known that catalytic activity is substantially improved by complexing the transition metal with a support such as MgCl.sub.2. See, for example, Barbe, P. C., Cecchin, G., Noristi, L., "The Catalytic System Ti-Complex/MgCl.sub.2 ", Advances in Polymer Science (1987), 81, pp. 1-81. Typically, a quantity of MgCl.sub.2 and one or more titanium procatalysts are mixed together in a nonpolar solvent to yield a material capable of polymerizing olefins in the presence of a cocatalyst such as an aluminum alkyl.
It is also known that it is possible to very substantially increase the catalytic activity of the aforementioned reduced transition metal compounds by means of a preactivation treatment. This treatment involves contacting the transition metal compound with magnesium and one or more alkyl halide(s). The reduced transition metal compounds which are preactivated by this treatment result in catalysts which make polymers having good physical characteristics and capable of being processed by injection molding or by extrusion. By virtue of the high degree of activity of the preactivated catalysts, removing the catalytic residues contained in the polymers becomes unnecessary.
U.S. Pat. No. 4,042,771, issued to Michel Avaro and Pierre Mangia on Aug. 16, 1977, discloses one type of preactivation treatment. In Avaro's treatment, the magnesium used was in the form of powder or turnings because it is said to be preferable to have the magnesium in a high state of purity. In order to facilitate preactivation of the solid transition metal compounds, the magnesium is used in a reactive form which is substantially devoid of impurities due in particular to oxidation of the metal. Avaro et al. state that, in practice, the magnesium in the industry is activated before being introduced into the medium in which preactivation is effected. According to Avaro et al., previous activation of the magnesium can, for example, comprise grinding the metal in an inert atmosphere or in an inert liquid such as an aliphatic solvent. This preliminary operation can also be effected by treating the magnesium with iodine. It is stated to be more convenient, however, to activate the magnesium within the medium in which preactivation is effected.
There are several problems with the procedure disclosed by Avaro et al. For one, the reaction depends on the magnesium source and its state of purity. Therefore, reproducibility of experimental results can be a problem. Also, when the procedure involves the added step of grinding the magnesium, additional equipment is required. Furthermore, when iodine is used to activate the magnesium, unreacted magnesium and iodine may contaminate any subsequent reactions. Lastly, catalysts prepared by Avaro et al.'s procedure produce unacceptable amounts of fine polyolefin particles, i.e., particles of polyolefin having a diameter of less than 180 microns. Consequently, an undesirable elutriation of the catalyst is required to remove fine catalyst particles before polymerization can occur in a commercial reactor.
Use of a preformed alkylmagnesium halide as a magnesium source in the preactivation treatment of olefin polymerization catalysts is also known in the art and solves some of the aforementioned problems. U.S. Pat. No. 4,355,143, issued to Lassalle on Oct. 19, 1982, discloses the use of organomagnesium halide in the preactivation treatment of catalysts used for the polymerization of olefins.
Lassalle discloses that the catalyst may be preactivated by reaction of one or more compounds of tetravalent titanium, and an organomagnesium halide compound having the formula MgXR or the formula MgR.sub.2 wherein X is a chlorine or bromine atom, and R is an alkyl radical which may contain from 2 to 8 carbon atoms.
Example C of Lassalle's patent discloses a procedure for preparing the catalyst. In that example, a preformed alkylmagnesium halide is first prepared by reacting powdered magnesium in a flask with an alkyl halide in heptane with an iodine crystal. A solution of titanium compounds is added to the resulting suspension of alkylmagnesium halide over a period of 2 hours, and the resulting product forms by precipitation.
It is advantageous to make a catalyst that minimizes the amount of fine particles of polyolefin that remain after polymerization. It is also advantageous to create a catalyst which can produce polyolefin having a higher melt index at lower concentrations of hydrogen. The present invention provides these advantages.